Method for controlling channel re-selection

ABSTRACT

A method for re-selecting control channels in a mobile station reduces the use of resources by limiting an evaluation of candidate control channels to those channels which satisfy certain criteria and are deemed to be eligible control channels. As a consequence, resources are not spent on attempting to re-select a control channel that is not compatible with the mobile station.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 10/921,822 filed Aug. 20,2004, U.S. Pat. No. 7,447,487, which is a continuation of applicationSer. No. 10/206,963 filed 30 Jul. 2002, U.S. Pat. No. 6,850,744, whichis a continuation of application Ser. No. 09/306,760 filed May 7, 1999,U.S. Pat. No. 6,501,951, which is a continuation of application Ser. No.08/772,829 filed Dec. 24, 1996, U.S. Pat. No. 5,995,834, the contents ofwhich are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention is directed to a method for controlling controlchannel re-selection by a mobile communication station. Moreparticularly, the invention relates to allowing the re-selection processto be conducted only with respect to those candidate control channels,onto which the mobile station is eligible to camp.

In the wireless communication environment it is known that as a mobilestation moves throughout an area it is likely to encounter differentcells that are serviced by different base stations. One example of amulti-cell wireless environment is illustrated in FIG. 1. There are aplurality of cells (C₁ to C₁₂) and each is served by a separate basestation (BS₁ to BS₁₂). As a mobile station moves from one cell toanother cell it is served by a different base station.

Typically the base stations operate with distinct control channels,voice channels or data channels. It is known that during acommunication, such as a call, as a mobile station passes from one cellto another it is handed off from one base station to another accordingto a certain protocol. However, in an activated state where a call isnot in process, while the mobile station is in a cell, it is camped on acontrol channel, i.e., it is in essence tuned to that channel andprepared to engage in communications of control information over thatchannel. The mobile station must execute a process for selecting anothercontrol channel as the mobile station moves from cell to cell. This iscalled a re-selection process.

In the wireless environment, a known standard referred to as the IS-136Standard defines protocols for digital wireless communications involvingdigital mobile stations. That standard defines a process by which themobile station can re-select a control channel as it moves among thecells. In that process the mobile station is presumed to be “camped-on”to a first control channel. That control channel then transmits a listof the neighboring control channels, that is the list of controlchannels for the neighboring cells (or service providers). The mobilestation receives the list and stores it. Then in accordance with aprescribed algorithm, the mobile station determines which one of thecontrol channels on the list constitutes the primary candidate forre-selection. This determination is made based on such things as readingthe RF (Radio Frequency) level of the various control channels in thelist and determining which is the control channel that the mobilestation should try to select next. For example, if the mobile stationwas in cell C₂ moving towards cell C₁, it may detect that the nextstrongest control channel among the neighbors of cell C₂ originates frombase station BS₁ and therefore would deem that control channel to be theprimary candidate for re-selection.

Further, in accordance with the algorithm described in the IS-136standard once a primary candidate has been determined the mobile stationattempts to re-select to that control channel. If that re-selectionattempt is successful, then the mobile station camps onto the newcontrol channel, receives a new list of neighboring control channels andbegins the process all over with the new list of neighbors. If theattempt is unsuccessful, then the mobile station either selects the nextappropriate candidate channel (that is a secondary candidate channel) orrepeats the analysis of the entire list of candidate channels attemptingto determine another primary candidate channel for re-selection.

A problem arises in that when the mobile station receives the neighborlist, it may include control channels with which the mobile station issimply incompatible. For example, the list may include one or morechannels that are related to private systems and a mobile station maynot be permitted access to those private systems. Alternatively, themobile station may be seeking voice service and the control channels maybe directed to data or fax services. Nonetheless, based on theparameters used in the analysis algorithm, these control channels withwhich the mobile station cannot truly interact are included in theanalysis operation and one of these incompatible control channels couldbe selected as the primary candidate control channel. Naturally whenthis occurs, the attempt to re-select to that control channel will fail.Thus, the execution of the process has wasted time and resources towardthe attempt to re-select an incompatible control channel. Furthermore,under the known algorithms, once the attempt to re-select to theincompatible control channel has failed that channel is still includedin the next cycle of the re-selection process and could be once againselected as the primary candidate control channel. It is conceivablethat the mobile station could get caught in an extended loop ofdetermining an incompatible control channel to be the primary candidatecontrol channel and making a failed attempt to re-select to that controlchannel.

In view of the shortcomings of the known re-selection process, it isdesirable to provide further controls in the re-selection process toavoid the attempt to re-select to incompatible control channels. It isalso desirable to avoid second attempts to re-select to control channelswhich are determined to be incompatible with the mobile station during are-select attempt.

SUMMARY OF THE INVENTION

In accordance with the present invention a mobile station performs amethod for controlling the re-selection of a control channel. In thatmethod the mobile station, prior to performing any algorithm forselecting a primary candidate control channel, marks as ineligible, anyone of the control channels identified on the neighbor list which areincompatible with the mobile station. The selection of a primary controlchannel then proceeds only with regard to those control channels whichare determined to be eligible. Once a primary candidate control channelis selected the mobile station attempts to re-select to that primarycandidate control channel. If the attempt to re-select is successful,then the mobile station camps on to this control channel. If, however,the attempt fails, then the mobile station determines why the attemptfailed. If the failure arises from an incompatibility between the mobilestation and the control channel, then that control channel is marked asineligible in the neighbor list and when the re-selection process isrepeated with the same neighbor list, since that control channel hasbeen marked as ineligible it will not be a factor in the selection of aprimary candidate control channel.

Examples of incompatibility are as follows. The mobile station may nothave any access privileges with regard to private systems. In this case,the mobile station would scan the neighbor list and mark as ineligibleany control channel identified as being related to a private systemsince the mobile station will not be permitted access to such systems.Alternatively, the mobile station may have access to only one privatesystem. In such a case, then if a primary candidate control channel is aprivate system channel and the mobile station is not determined to haveaccess to that particular private system, then the attempt to re-selectto the private system will fail and the neighbor list will be modifiedto reflect that fact that this channel is not eligible for analysis in asubsequent re-selection process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates in a high level schematic form a wireless system inwhich the present invention can be employed.

FIG. 2 illustrates a table of information employed by a mobile stationin the environment of FIG. 1 to implement the present invention.

FIG. 3 illustrates a process flow for reselection of a control channelin the wireless channel of FIG. 1 in accordance with an embodiment ofthe present invention.

DETAILED DESCRIPTION

As described above, the mobile station which employs the presentinvention may be used in a wireless embodiment which is shownschematically in FIG. 1. A cellular phone providing voice service couldconstitute such a station. A plurality of cells C₁ to C₁₂ are servicedby respective base stations BS₁ to BS₁₂. When the mobile station is incell C₁, it may be camped on to the control channel from base stationBS₁. The control channel BS₁ will transmit a list of neighboring controlchannels. The list will include control channels associated with basestation BS₂, base station BS₅ and base stations that service other cellsadjacent to cell C₁. Furthermore, the list of neighboring channels mayinclude a list of control channels that are related to fax services ordata services. In addition, the list may include a control channelassociated with a private system such as that shown in cell C₁.Typically such a private system is overlaid by the public system so thatthe area covered by the private system is covered by both the public andprivate system, but access to the private system itself, for example, awireless PBX on a party's premises, is not permissible except by membersof that private system.

When the mobile station receives the neighbor list from the controlchannel on which it is camped, it stores that neighbor list in memory.The mobile station may be a cellular phone which complies with theIS-136 standards. Such a mobile communications device includesprocessing capabilities and memory, both read only memory (ROM) andrandom access memory (RAM). The ROM stores control programs foroperating the device while the RAM stores dynamic information which canbe updated over time, such as the neighbor list transmitted from acontrol channel on which the station is camped. In its memory the mobilestation would track the identification of the control channels includedin the list. Furthermore, in compliance with the IS-136 standard, theneighbor list would also provide certain parameter information whichrelates to the characteristics of the control channel. For example, theparameter information would indicate whether a control channel isassociated with a private, a public, or a semi-private system. Theidentification of the particular system to which the control channel isassociated would not necessarily be presented in the neighbor list.Instead only the type of system that the control channel is associatedwould be supplied. Similarly, the parameter information could indicatewhether the control channel is designed to provide voice service, dataservice, or fax service. Other parameter information which defines thecharacteristics of the control channel could also be supplied with aneighbor list, (e.g., cell type, protocol version, cell sync, etc.).

An example of a correlation of the information presented by the controlchannel in which the mobile is camped, is shown in FIG. 2. In thistabular representation of the data which might be stored in the mobilestation, a first control channel in the list has an ID of “XXXXX”. Theparameter information indicates that this control channel is associatedwith a private system and provides voice services. The control channelidentified by “YYYYY” is by contrast associated with the public systemwhile still providing voice services. The control channel “ZZZZZ” isalso associated with the public system, but provides fax services.Similar information would be provided for each control channelidentified in the neighbor list provided by the control channel on whichthe mobile station is camped.

This parameter information can then be used to modify or control theprocess by which an alternative control channel can be selected.

A flow chart illustrating the process for controlling re-selection usingthe received neighbor list is illustrated in FIG. 3.

In step 300, the mobile station receives the neighbor list from thecontrol channel on which it is presently camped. In Step 301, the mobilestation processor scans the neighbor list in accordance with a storedcontrol program and examines the entries in the list to determinewhether any of the control channels (the candidate control channels) areineligible because of a lack of compatibility between the mobile stationand the control channel. As an example, incompatibility could arisewhere the mobile station does not have access to any private network orsystem. The mobile station memory would store system IDS for thosesystems with which the mobile station is allowed to communicate. Itcould also store a flag indicating whether the mobile station has accessto any private system. Under those circumstances where there is accessto private systems, any control channel which is related to a privatesystem is incompatible with the mobile station. Similarly, if the mobilestation seeks voice services, then any control channel associated withfax or data services would be incompatible with the mobile station.

Once a candidate control channel is detected or recognized to beineligible in Step 302, then each of those ineligible control channelsis marked as ineligible in the neighbor list. In particular, theprocessor in the mobile station modifies the neighbor list to somehowmark a control channel as ineligible. One way of doing this is toinclude an eligibility flag in the neighbor list. All control channelsin the neighbor list would initially have their eligibility flags set asindicating an eligible control channel. Then, when a control channel ismarked as ineligible the flag would be reset to an ineligible state.Alternatively, it is possible that other steps could be taken toeffectively remove the ineligible control channels from subsequentconsideration in the re-selection process. In Step 304, a process, knownin the prior art, is initiated for studying or analyzing candidatechannels to try to select the optimal candidate channel for reselection.This process is referred to in FIG. 3, as “analyzing candidate controlchannels.” That step of analysis is limited to only those controlchannels which are deemed to be eligible. That is, if the eligibilitystatus reflected in the neighbor list in Step 303 indicated that acontrol channel is ineligible, then that control channel will not beincluded in the analysis operation beyond that point. It will, inessence, be ignored and will not figure in the calculations ofdetermining the best candidate control channel for the re-selectionprocess. In the analysis operation, each of the eligible controlchannels will be tested for certain criteria such as RF level. Once eachof the eligible control channels is tested a primary candidate channelis selected (step 305) as the processor identifies which of the eligiblecontrol channels is the best candidate for re-selection based on thetest results obtained during the analysis of step 304. This may ariseunder the circumstance where the processor will run tests on each of theeligible control channels. As an example a result of the tests on suchthings as the RF level in step 304 the processor may determine that oneor more of the candidate control channel satisfy certain criteria to beselectable by the mobile station. Then, in step 305 the processor couldgo through this subset of selectable candidate control channels andselect the optimal or primary candidate control channel based on theparameters associated with that control channel. Thus, a primarycandidate channel could be as in step 305. Once such a primary candidatechannel is identified, the mobile station attempts to re-select to theprimary candidate channel in Step 306. If the re-selection attempt issuccessful as detected in Step 307, then the mobile station camps ontothe primary candidate channel in Step 308 and receives a new neighborlist from the primary candidate control channel identifying neighborsassociated with that control channel. If, however, the attempt tore-select is unsuccessful, then the processor can make a determinationas to why the reselection attempt failed. If the failure is due to amismatch of certain predetermined criteria as referred to in Step 309,then the channel identified as the primary candidate control channelcould then be treated as an ineligible candidate channel. Theeligibility status within the neighbor list would be modified to reflectthis change of status and the processor could then resume there-selection process from Step 304, where the processor could beginagain the analysis of the candidate control channels focusing only onthose which remain as eligible control channels. The system will thenselect another primary candidate and attempt to reselect to that secondprimary candidate channel. This process will continue until the mobilestation camps onto an alternative control channel.

As has been described above, a candidate channel could be ineligiblebecause the control channel is associated with a private system, whereasthe mobile station is not affiliated with any private system.Furthermore, a control channel may be deemed ineligible because of thetype of service that it provides and the lack of compatibility betweenthat service and the service of the mobile station. The predeterminedcriteria referred to in connection with Step 309 can be any conditionthat causes the failure of a reselect attempt. Typically one such eventwould be where the primary candidate channel is associated with aprivate system and the mobile station is also associated with a privatesystem. In that circumstance, then, the primary candidate channel wouldnot then be marked ineligible in Step 303. Then, during the reselectattempt the private system identifier associated with the primarycandidate channel would be provided to the mobile station. If the mobilestation private system identifier stored in memory does not match theprivate system ID received from the primary candidate channel during theattempt to re-select, then the mobile station will not get access to theprivate system associated with that primary candidate channel.Therefore, there is an incompatibility between the mobile station andthe system associated with that primary candidate channel. In view ofthis incompatibility, it is consistent with the present invention to nowmark this primary control channel as ineligible as in step 310. Then, ifit is necessary to analyze the neighbor list again (step 304) to find acandidate channel for re-selection, the newly designated ineligiblecontrol channel will not be analyzed.

In accordance with the present invention, other parameter informationassociated with a control channel could be used to determine whether thecontrol channel is eligible for re-selection by this particular mobilestation. Furthermore, modifications to the process of FIG. 3 are alsopossible. For instance, in one variation Steps 301, 302 and 303 would beeliminated, that is the processor would not do any preliminaryexamination of the neighbor list to determine if any candidate channelsare ineligible. Instead, the system would simply rely on marking primarycandidate channels as ineligible for subsequent reselection attempts.Alternatively, the control method of the present invention could relysimply on the ineligibility determination made at the beginning of theanalysis process and not dynamically evaluate ineligibility based onwhether an attempt to reselect a particular candidate channel wassuccessful. Thus steps 309 and 310 could be eliminated and benefitswould still be obtained from the remaining process.

Since the present invention resides in a method by which the mobilestation selects an appropriate reselection candidate and since it isimplemented using software running on a processor within the mobilestation, it must be recognized that variations on the order in whichcertain steps are performed and the specific techniques or parametersinvolved in the process could be modified while still falling within thespirit of the present invention. For example, it is conceivable that inone variation the station microprocessor could scan the entire list ofneighbors and adjust the eligibility status where appropriate for allineligible control channels before proceeding with the analysis of theeligible control channels. In an alternative embodiment, the first timethrough the neighbor list the processor could first determine whether agiven control channel is eligible and then, if it is eligible, performthe analysis with respect to that control channel. Then themicroprocessor would turn to the next control channel on the list,determine whether it should be considered eligible and if so conduct theevaluation with respect to that control channel and so on. In thissecond configuration, the processor does not scan the entire list beforebeginning the evaluation process. Instead, it combines the marking andevaluation steps.

Other modifications might include other techniques for marking a controlchannel as ineligible.

In yet another embodiment it is conceivable that the mobile stationcould select for analysis a subset of the control channels from theneighbor list; the subset being based on any one or combination ofparameters.

In accordance with the present invention, a mobile station can moreoptimally control the process by which it selects alternative controlchannels. It provides the mobile station with a way to focus only onthose candidate control channels with which the mobile station caneffectively communicate.

1. A mobile station comprising: a processor; and a first memory storingprogram instructions which, when executed by the processor, cause theprocessor to: receive a list of candidate control channels at the mobilestation over a control channel on which the mobile station is currentlycamped; analyze said list to determine a primary candidate channel forre-selection; attempt to re-select said primary candidate channel; andif the attempt to re-select fails because the primary candidate channelcorresponds to a private system to which the mobile station is notpermitted access, then marking the primary candidate channel asineligible for re-selection such that during a subsequent attempt tore-select the control channel marked as ineligible is not considered. 2.The mobile station of claim 1, wherein the program instructions which,when executed by the processor, further cause the processor: determineif a candidate control channel has been marked ineligible; and consideronly those control channels that have not been marked ineligible.
 3. Themobile station of claim 1, wherein the program instructions which, whenexecuted by the processor, further cause the processor: determine if acandidate control channel is marked as private in said list of potentialcontrol channels; and consider said control channel for re-selectiononly if said mobile station has a private system identification storedin a memory.
 4. The mobile station of claim 1, wherein the programinstructions which, when executed by the processor, further cause theprocessor: to deem a control channel as incompatible when there is amismatch between a service it provides and a service sought by themobile station.